# Implement a custom logger

Many applications must integrate with specific remote logging solutions such as Honeycomb and Azure Application Insights. To facilitate this integration, the shell runtime accepts any custom loggers implementing the Logger interface.

# Create a custom logger class

First, let's define a custom logger:

host/src/customerLogger.ts
import { Logger, type LogLevel, type Runtime } from "@squide/firefly";

export class CustomLogger implements Logger {
    readonly #logLovel: LogLevel

    constructor(runtime: Runtime, logLevel: LogLevel = LogLevel.debug) {
        super(CustomLogger.name, logLevel);

        this.#logLevel = logLevel;
    }

    debug(log: string, ...rest: unknown[]): Promise<unknown> {
        if (this.#logLevel <= LogLevel.debug) {
            console.log(`[custom-logger] ${log}`, ...rest);
        }

        return Promise.resolve();
    }

    information(log: string, ...rest: unknown[]): Promise<unknown> {
        if (this.#logLevel <= LogLevel.information) {
            console.info(`[custom-logger] ${log}`, ...rest);
        }

        return Promise.resolve();
    }

    warning(log: string, ...rest: unknown[]): Promise<unknown> {
        if (this.#logLevel <= LogLevel.warning) {
            console.warn(`[custom-logger] ${log}`, ...rest);
        }

        return Promise.resolve();
    }

    error(log: string, ...rest: unknown[]): Promise<unknown> {
        if (this.#logLevel <= LogLevel.error) {
            console.error(`[custom-logger] ${log}`, ...rest);
        }

        return Promise.resolve();
    }

    critical(log: string, ...rest: unknown[]): Promise<unknown> {
        if (this.#logLevel <= LogLevel.critical) {
            console.error(`[custom-logger] ${log}`, ...rest);
        }

        return Promise.resolve();
    }
}

Then create a FireflyRuntime instance configured with an instance of the new CustomLogger:

host/src/bootstrap.tsx
import { FireflyRuntime } from "@squide/firefly";
import { CustomLogger } from "./customLogger.ts";

const runtime = new FireflyRuntime({
    loggers: [x => new CustomLogger(x)],
});

# Try it 🚀

Start the applications and open the developer tools, then, refresh the page. You should see the following console log message:

> [custom-logger] [shell] Found 1 remote modules to register.